1. Field of the Invention
The field of the invention is that of touch-sensitive or ‘touchscreen’ surfaces with capacitive detection and more particularly ‘multi-touch’ touch-sensitive surfaces for detecting two simultaneous presses. This function is essential for ‘zooming’ or image rotation, for example. The more specific field of the invention is that of detecting faults on said touch-sensitive surface. This invention can be applied to different uses but is particularly well suited to the constraints of the field of aeronautics and aircraft instrument panels where detecting malfunctions is essential for ensuring flight safety.
2. Description of the Prior Art
‘Projected’ capacitive detection consists in implementing a detection matrix composed of conductive rows and columns arranged so as to detect local variations in capacitance introduced by the proximity of the user's fingers or any other conductive pointing object. ‘Projected capacitive’ technology comes in two main variants, which are:                ‘Self capacitive’ detection which consists in reading the rows then the columns of the matrix touch network;        ‘Mutual capacitive’ detection consisting in reading each intersection of the matrix touch network.        
‘Mutual capacitive’ technology requires reading the entire panel.
Thus, if the matrix comprises N rows and M columns, N×M acquisitions must be made, making it problematic to implement large, high resolution panels with low response times. In addition, the ability to measure in ‘Mutual capacitance’ is weaker than that obtained in ‘Self capacitive’ detection, which makes it problematic for the user to use gloves.
The advantage of ‘Self capacitive’ detection is that, for the preceding panel, the system requires only N+M acquisitions for reading the matrix. FIG. 1 illustrates this principle. In this FIG. 1, a first finger is pressing on a first intersection of column CI and row LJ and a second finger is pressing on a second intersection of column CK and row LL. The output voltages VOUT of the rows and columns display easily identifiable drops in level. Measurements of the voltages around each drop in level can be used to accurately identify the rows and columns required.
However, this latter technique has a drawback. This is because the press or presses is/are detected with the aid of a network of conductive and transparent rows and columns incorporated into a glass substrate. The loss of a row or column creates an unusable dead zone. This defect is all the more serious in that it appears only when the user needs to use the dead zone. It is therefore a dormant fault undetected by the system. In a product intended for mass consumer applications, such as a touch pad, this defect is not necessarily very troublesome, but on the other hand, it becomes very serious in certain technical fields such as aeronautics where the requirements of reliability are very important and in which it is essential to ensure the availability of the system at least until the end of the flight or mission.
This technique has another drawback. It is not always easy to assign the rows and columns detected to the correct intersections actually touched by the user's fingers. Possible intersections not actually touched are generally called ‘ghosts’. For countering this last difficulty, the applicant has developed a technique consisting in performing a scan of the matrix at two different acquisition frequencies. This technique is described in the publication ‘Eliminating Ghost Touches on a Self-Capacitive Touch-Screen’ that appeared in ‘SID 2012 DIGEST’ of June 2012.